Southwest and AirTran aircraft at gates at San Francisco International Airport on Nov. 8, 2010. On May 2, 2011, the airlines closed on their planned merger. / By Ben Mutzabaugh, USA TODAY

by Ben Mutzabaugh, USA TODAY

by Ben Mutzabaugh, USA TODAY

Southwest Airlines remains the only big U.S. carrier that doesn't charge customers for changing their flight plans, but the carrier said today that it will tighten its rules around that customer-friendly policy.

Beginning sometime next year, Southwest will add a "no-show" fee that will apply customers who miss a flight but fail to cancel their seat prior to takeoff.

Currently, Southwest imposes no penalty in that scenario, allowing no-show fliers to apply the full value of their tickets from a missed flight toward the purchase of a new one.

But Southwest says the number of customers who do that are now leaving too many empty seats on its flights. It will respond by adding the no-show fee to its cheapest "Wanna Get Away" fares sometime in 2013, though the company has not decided the exact price of the fee or exactly when it will go into effect.

The news came out of an investor conference call earlier today (Friday) from New York that included many of the top officials from Southwest and subsidiary AirTran.

Among other details from the call: Southwest said it plans to start selling some itineraries that involve connections to or from subsidiary AirTran next month and detailed an increase in some existing fees for both it and AirTran.

Most of the media attention from the call, however, focused on the no-show fee.

"We have made the decision to tighten the flexibility around our most restrictive tickets," Southwest CEO Gary Kelly is quoted as saying by The Dallas Morning News.

"Specifically, we will implement a no-show fee for reuse of funds associated with restrictive tickets that are not flown and not cancelled by our passengers prior to flights," Kelly adds, according to the Morning News. "This should add ancillary revenue and promote customer behavior that allows us to sell the open seat prior to departure."

Kelly said the no-show fee will make its polices more like those at its more-traditional "legacy" rivals, but hopefully in a way that will not alienate customers of the carrier that bills itself as the USA's "no-fee" carrier.

"By our research, customers understand that we all could benefit - customers and the company - from the opportunity to resell a seat," Kelly is quoted as saying by The Associated Press. "Once the airplane takes off and (a seat) is empty, we can't ever reclaim that."

Merger integration with AirTranSouthwest also gave more details on its timeline for integrating its operations with subsidiary AirTran, which it acquired in 2011.

The Wall Street Journal writes Southwest "plans to start code-sharing, or cross-selling seats, with the AirTran business for the first time early next year â?¦ ."

Southwest will start by linking only a few of the carriers' flights in January, gradually adding more flight options through the winter and spring. The complete codesharing of the entire combined Southwest and AirTran networks should come by April â?? "presuming everything goes well," CEO Kelly is quoted as saying by the Morning News.

The Atlanta Journal Constitutionwrites "that will trigger more flight schedule changes, on top of the AirTran routes already canceled since the acquisition. Southwest has closed AirTran operations at 17 airports so far."

Southwest says it hopes to have fully integrated AirTran's operations into its own brand by 2014.

In other news from today's investors call:

Southwest said it will institute a program to sell priority boarding slots at the gate. Currently, customers can buy the carrier's $10 "Early-Bird Check-In" option, which guarantees purchasers to be among the first to board the plane. Southwest which does not assign seats, but it does assign customers numbered spots in the boarding queue. Customers then can pick any open seat still left when they board the plane.

Southwest will boost the price of its Early-Bird Check-in option from $10 to $12.50, according to BusinessWeek.

AP writes: "In recent years, Southwest has bombarded TV viewers with 'bags fly free' commercials to highlight that it doesn't charge customers for their first two checked bags or for changing a reservation - both fees are standard on most other major U.S. airlines. Southwest executives said they're not thinking about imposing those fees, but would announce other changes Saturday."

Fees for overweight bags will rise from $50 to $100.

Southwest's fee for a third bag â?? currently $50 â?? will rise to an unspecified amount.

AirTran's bag fees will rise to $25 for a first bag (up from $20) and to $35 for a second (up from $25). Despite the increase, Southwest has said it will end AirTran's fees once AirTran is fully assimilated into the Southwest network.

Southwest said it expects to add $100 million in revenue in 2013 by better utilizing its aircraft.